1. Field of the Invention
The invention relates in general to wireless communication, and more particularly to a method and technique for mitigating frequency pulling for a voltage-controlled oscillator (VCO).
2. Description of the Related Art
In wireless communication, a signal to be transmitted is basically generated in a signal having a relatively low frequency. The relatively low frequency is commonly referred to as baseband. With a certain process, the baseband signal is attached in a radio-frequency (RF) signal having a relatively high frequency and transmitted. Such process is referred to as up-conversion, which is performed by a transmitter in an RF transceiver. Conversely, an opposite process is referred to as down-conversion, which is performed by a receiver in the RF transceiver. Both up-conversion and down-conversion require local oscillation (LO) signals having correct phases. The LO signals can be generated by a voltage-controlled oscillator (VCO) having a good oscillation stability and associated circuits.
The oscillation stability of a VCO may be interfered by normal operations of nearby devices. Such interference is substantially categorized into two types—frequency pushing and frequency pulling. Frequency pushing is a frequency change in a signal of the VCO caused by an unstable voltage of a power line or a ground line of the VCO. Factors incurring the frequency pulling may be a rush current in the nearby components of the VCO, or a coupling effect generated by parasitic resistance, capacitance or inductance of a power line or a ground line. On the other hand, frequency pulling is an effect imposed on an operating frequency of the VCO caused by a large-energy RF signal or harmonics of an RF signal through interactions of electric, magnetic or electromagnetic coupling.
FIG. 1 shows a direct conversion transceiver 100 comprising a transmitter 12 and a receiver 16. Under efficiency considerations, the transceiver 100 only comprises a frequency synthesizer 14 for providing in-phase/quadrature RF signals SI and SQ to be shared by the transmitter 12 and the receiver 16. In other examples, the transmitter 12 and the receiver 16 may respectively have a frequency synthesizer.
The transmitter 12 transmits a message in a digital-bit signal to a digital logic circuit 18. In this example, the digital logic circuit 18 may be multi-functional, e.g., being capable of providing debug computing of a communication signal by being equipped with additional digital bits. For example, the digital logic circuit 18 is also capable of generating quadrature modulation signals according to the digital-bit signal received, i.e., signals A(n)cos(θ(n)) and A(n)cos(θ(n)+π/2). Wherein, A(n) and θ(n) are determined by a modulation type (e.g., phase-shift keying (PSK), frequency-shift keying (FSK) or amplitude-shift keying (ASK)) to be performed by the transmitter 12. Throughout the specification, two quadrature signals refer to two signals with a difference of π/2 radians or a 90-degree phase.
One of the two modulation signals is sent to an in-phase transmission path while the other is sent to a quadrature-phase transmission path. It is observed from FIG. 1 that, the digital logic circuit 18 ensures that a difference of π/2 radians or a 90-degree phase exists between the two digital signals on the two paths. On each of the transmission paths, a digital-to-analog converter (DAC) 20 converts the corresponding digital-bit modulation signal sent from the digital logic circuit 18 to an analog modulation signal. The analog modulation signals generated by the DACs 20 are filtered by low-pass filters 22. The filtered analog signals are then ready to be blended by a mixer 24 with an RF signal provided by the frequency synthesizer 14 and up-converted to RF.
The frequency synthesizer 14 provides the two quadrature RF signals (with a difference of π/2 radians) SI and SQ to the two mixers 24 on the in-phase transmission path and the quadrature transmission path, respectively. Results generated by the two mixers 24 are combined by an adder 28 and the combined signal is forwarded to a power amplifier 26 to boost signal strength of the combined signal. The signal processed by the power amplifier 26 is then transmitted to the air via an antenna 30.
In the transceiver 100, the two RF signals SI and SQ provided to the mixers are generated by a phase-locked loop (PLL). A phase detector 32 compares a reference signal fref with a feedback signal generated by the frequency synthesizer 14. Thus, an output signal of the phase detector 32 corresponds to a phase difference between the reference signal fref and the feedback signal, and is processed by a low-pass filter 34 to generate a control voltage Vctrl.
In FIG. 1, a VCO 36 in an oscillation signal generator 35 generates a high-frequency signal having a corresponding oscillation frequency according to the control voltage Vctrl. A divider 38 with a divisor of 2 frequency divides the high-frequency signal generated by the VCO 36, and provides the in-phase RF signal SI and the quadrature signal SQ that are quadrature to each other to the transmitter 12 and the receiver 16. One of the RF signals SI and SQ is frequency divided by the divider 40 having a divisor of N to generate a feedback signal.
The RF signals SI and SQ are respectively sent to the mixers 24 on the in-phase transmission path and the quadrature transmission path. The blended results are combined by the adder 28 and then processed by the power amplifier 26 for reinforcing the signal strength. It is known from FIG. 1, assuming the RF signals SI and SQ are respectively cos(wt) and cos(wt+π2), the large-power RF signal outputted by the power amplifier 26 probably included a cos(wt+θ(t)) component. Also known from FIG. 1, assuming the RF signals SI and SQ are respectively cos(wt) and cos(wt+π2), the high-frequency signal generated by the VCO 36 may be represented as cos(2 wt).
Since the fundamental frequency of the large-power RF signal outputted by the power amplifier 26 is w, the harmonic frequency (i.e., an integral multiple frequency of the fundamental frequency) of the large-power RF signal inevitably contains a considerable amount of energy. In FIG. 1, the oscillation frequency (2 w) of the VCO 36 is coincidently the same as one of the harmonic frequencies outputted by the power amplifier 26. As a result, in case of any leakage of the large-power RF signal outputted by the power amplifier 26, the leakage energy becomes a spurious signal. The spurious signal reaches the VCO 36 through the antenna 30 or the electromagnetic coupling effect in the transceiver 100 and to impose pulling effects on the phase of the VCO 36, such that the oscillation stability of the VCO 36 is depreciated.